Blood is mostly water with some salt and minerals; not cleaning it off is a great way to ruin a blade, especially carbon steel (it'll also do a number on the leather the sheath is made of). Many years ago, my uncle wrecked a German officer's dagger he had taken as a souvenir that way.

When it became obvious that the Germans were breaking through, a lot of third-line troops (clerks, cooks, et cetera) were handed rifles and sent into the line. Most of these guys hadn't handled weapons since Basic Training.
My uncle was a tank mechanic (in the 3rd, IIRC).